1. Field of the Invention
The present invention relates to a photosensitive insulating paste suitable for forming an insulating layer in, for example, a high-frequency circuit, and to a thick-film multi-layer circuit substrate containing the paste.
2. Description of the Related Art
As high-frequency electronic apparatuses, including portable terminals and computers, have come to have a higher density and a higher signal speed, insulating materials used in high-frequency circuits or like elements of the apparatuses are required to have lower relative dielectric constants and higher Q values. Such insulating materials are primarily used for forming an insulating layer that separates two or more electrodes or transmission lines in, for example, a high-frequency circuit substrate or an electronic component for a high-frequency circuit.
Generally, the insulating layer is formed of an insulation paste containing glass powder or a similar material. The insulating layer often contains via holes in the form of micro-pores for electrically connecting electrodes and circuits, such as those formed by transmission lines, which are disposed on the upper and lower surfaces of the layer. The insulating layer containing via holes is formed through the following procedure, for example. An insulating paste is applied onto a substrate by way of printing at a predetermined position by means of a technique such as screen printing, and the paste is dried. Subsequently, via holes are formed in the paste and the paste is fired to thereby form the insulating layer.
A technique making use of a photosensitive insulating paste is also known as a technique for forming via holes in a dried insulating paste. In the technique, an insulating paste containing a photosensitive resin such as a photosensitive polyimide resin is used and micro-via holes are formed in the paste by means of photolithography.
For example, Japanese Patent Application Laid-Open (kokai) Nos. 110466/1997 and 50811/1996 disclose a process for forming micro-via holes in an insulating paste. In the process, a glass material containing SiO2, Al2O3, B2O3, Bi2O3 and ZnO or a glass material containing Bi2O3, SiO2, B2O3, BaO and ZnO is dispersed in a photocurable organic vehicle to thereby obtain a photosensitive insulating paste, the paste is applied to a substrate and the via-holes are formed by means of a photolithography technique in electronic elements for forming circuits which require multi-layer wiring.
Examples of insulating materials used for forming the above-described insulating layer include a resonator material such as Baxe2x80x94Ndxe2x80x94Pbxe2x80x94Tixe2x80x94O having a high Q value in a high-frequency region and an alumina-borosilicate glass ceramic material having a low dielectric constant.
However, the technique making use of screen printing may cause some problems, such as bleeding due to the viscosity of the insulating paste and poor printing resolution attributable to a printing plate. Therefore, an insulating layer containing micro-via holes having a diameter of less than 150 xcexcm is difficult to form, and thus, a circuit substrate and electronic components for a circuit have not fully satisfied the demand for reduction in size and enhancement of density.
Meanwhile, the technique for forming an insulating layer through a photolithography technique by use of a photosensitive insulating paste enables formation of micro-via holes having a diameter of less than 150 xcexcm. However, the thus-obtained insulating layer has poor durability at high temperature and low plating-resistance to an acidic plating solution.
Particularly, glass materials disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 110466/1997 and 50811/1996 are difficult to form into an insulating layer having high reliability when an Ag-type conductive material is used for forming transmission lines or electrodes adjacent to an insulating layer formed of the glass materials, since the Ag-type conductive material is highly reactive with the insulating layer. In addition, an ion cross-linking reaction may occur between the organic binder and a glass powder. For example, even when a glass powder is stabilized by use of a triazole, the viscosity of a photosensitive insulating paste may decrease with passage of time and consistent patterns may not be formed.
Meanwhile, a resonator material such as Baxe2x80x94Ndxe2x80x94Pbxe2x80x94Tixe2x80x94O has a high Q value but has a relatively high relative dielectric constant (∈r). Further, materials having a low dielectric constant, typified by an alumina-borosilicate glass ceramic material, has a limitation in terms of increasing Q value.
To overcome the above described problems, preferred embodiments of the present invention provide a photosensitive insulating paste which enables formation of very small via holes (hereinafter referred to as micro-via holes) as well as a film in a stable manner.
Preferred embodiments of the present invention also provide a thick-film multi-layer circuit substrate containing the above-described paste, which substrate is highly reliable and has an insulating layer having a high Q value and a low relative dielectric constant, wherein the density of circuit patterns is increased.
Accordingly, in a first aspect of the present invention, there is provided a photosensitive insulating paste comprising a borosilicate glass powder and a crystalline SiO2 powder, which powders are dispersed in a photosensitive organic vehicle, wherein the paste contains crystalline SiO2 in an amount of about 3-40 wt. % after sintering.
Preferably, the borosilicate glass contains SiO2, B2O3 and K2O, wherein the compositional proportions of the three components represented by (SiO2, B2O3, K2O) fall within a region formed by connecting points A (65, 35, 0), B (65, 20, 15), C (85, 0, 15), and D (85, 15, 0) in the ternary diagram shown in FIG. 1.
Preferably, the borosilicate glass powder and the crystalline SiO2 powder have a mean particle size of 0.1-5 xcexcm.
Preferably, the organic vehicle contains an organic binder, a photopolymerization initiator and a photocurable monomer, and the organic binder is an acrylic copolymer having a carboxyl group and an ethylenic unsaturated group in side chains of the copolymer. Preferably, the acrylic copolymer is an acrylic/modified-cellulosic copolymer. Preferably, the acrylic/modified-cellulosic copolymer has a compositional ratio of about 3/1-10/1.
In the photosensitive insulating paste as described in relation to the first aspect of the present invention, when the ratio of surface area to weight is represented by SS (m2/g), the ratio of surface area to volume is represented by CS (m2/cc) and specific gravity is represented by xcfx81, the borosilicate glass powder and the crystalline SiO2 powder have a shape-smoothness index (xcfx81xc3x97SS/CS) of about 1.0-3.0.
In a second aspect of the present invention, there is provided a thick-film multi-layer circuit substrate comprising an insulating substrate and an insulating layer formed thereon through thick-film printing, wherein the insulating layer is formed through exposure, development and sintering, after application of the photosensitive insulating paste described in relation to any one of the features or embodiments described above.
In the thick-film multi-layer circuit substrate according to the second aspect of the present invention, the insulating layer is provided between a first conductive layer and a second conductive layer, which conductive layers being formed to have predetermined circuit patterns, wherein the insulating layer contains via holes electrically connecting the first conductive layer with the second conductive layer.
Since the photosensitive insulating paste as described in relation to the first aspect of the present invention comprises a borosilicate glass powder and a crystalline SiO2 powder, which powders are dispersed in a photosensitive organic vehicle such that the paste contains crystalline SiO2 in an amount of about 3-40 wt. % after sintering, the crystalline SiO2 powder contained in such a high amount has low reactivity with an organic binder and the like, and therefore the viscosity of the paste hardly changes with passage of time and a film can be formed in a consistent manner. In addition, micro-via holes can be formed on the film by means of a photolithography technique.
Also, since the thick-film multi-layer circuit substrate as described in relation to the second aspect of the present invention is formed by use of an insulating layer which is prepared from the photosensitive insulating paste of the present invention and which contains a crystalline SiO2 component in the above-described predetermined amount, the layer has a high Q value and a low relative dielectric constant in good balance. In addition, micro-via holes of good shape can be formed on the layer by means of a photolithography technique, and the densities of via holes and circuit patterns can be increased.